expose argument 'maxit' so that users can set the no. of iterations in metaMDS

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2447 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'r-forge-svn-local'